A United Nations photo shows a woman carrying a baby as people are transported from Bor to Juba on Dec. 22. / Rolla Hinedi, AFP/Getty Images

by John Bacon and Jim Michaels, USA TODAY

by John Bacon and Jim Michaels, USA TODAY

American citizens were safely evacuated from a war-torn South Sudan city on Sunday, one day after four U.S. troops were injured when their aircraft drew fire during a failed airlift attempt,the State Department said.

An unspecified number of Americans and citizens from other "partner nations" were flown Sunday from Bor, the scene of intense fighting for the past week, to the capital of Juba on United Nations and U.S. civilian helicopters, department spokesperson Jen Psaki said in a statement.

"The United States and the United Nations, which has the lead for securing Bor airport in South Sudan, took steps to ensure fighting factions were aware these flights were a humanitarian mission," the statement said. The flights took place in consultation with the embattled South Sudanese government.

About 380 Americans and 300 citizens of other nations have been evacuated in recent days to Nairobi and other locations on four chartered flights and five military aircraft, Psaki said. An unknown number of people have left on their own and an unknown number remain in Bor and surrounding rural areas.

"The U.S. government is doing everything possible to ensure the safety and security of United States citizens in South Sudan," the statement said. "We are working with our allies around the world to connect with and evacuate U.S. citizens as quickly and safely as possible."

For security reasons, specific plans were not being released, Psaki said.

"We strongly recommend U.S. citizens in South Sudan depart immediately," the statement said.

In a letter to Congress on Sunday, President Obama said the military aircraft with 46 people aboard came under fire as they approached Bor and departed without completing the mission.

"The purpose of this operation was to protect U.S. citizens, personnel, and property,'' the President wrote."As I monitor the situation in South Sudan, I may take further action to support the security of U.S. citizens, personnel, and property, including our Embassy, in South Sudan.''

Also Sunday, South Sudan's central government lost control of the capital of a key oil-producing state, the military said, as renegade forces loyal to a former deputy president seized more territory in fighting that has raised fears of full-blown civil war in the world's newest country.

Bentiu, the capital of oil-rich Unity state, is now controlled by a military commander loyal to former vice president Riek Machar, said Col. Philip Aguer, the South Sudanese military spokesman.

A day earlier, the Pentagon said three aircraft were attacked by small-arms fire from the ground as they were approaching the town of Bor. All three aircraft sustained damage in the attack.

The aircraft, tilt-rotor Ospreys, were able to divert to Entebbe, Uganda, where the wounded American servicemembers were transferred to Air Force C-17 planes and flown to Nairobi, Kenya, for treatment. The four were in stable condition, the military said. The U.S. forces were part of Combined Joint Task Force-Horn of Africa.

South Sudanese officials blamed the attack against the American aircraft on rebels. The U.S. military identified the ground fire as coming from "unknown forces."

The country has been racked by violence for a week after an attempted coup triggered fighting between rival ethnic groups. The violence has killed hundreds and has world leaders worried that a full-blown civil war could ignite in South Sudan. Earlier this week, President Obama dispatched U.S. troops to help protect the U.S. Embassy in the capital, Juba.